Spectroscope-prism with curved surfaces.



0. PERY. SPBGTROSOOPE PRISM WITH GURVED SURFACES.

APPLICATION FILED MAY 19, 1910.

Patented Oct. 31,1911.

l/VVENTOR: a M

W/T/VESSES:

CHARLES FERY, OF PARIS, FRANCE.

SPECTROSCOPE-PRISM WITH CURVED SURFACES.

Specification of Letters Patent.

Application filed May 19, 1910. Serial No. 562,310.

To all whom it may concern:

Be it known that I, Cnamacs Friar, a citizcn of the Republic of France,residing in Paris, France, have invented certain new and usefulImprovements in Spectroscope- Prisms with Curved Surfaces, of which thefollowing is a specification.

In the invention a prism with curved surfaces is used to give a purespectrum without the aid of any concentrating system, such as convexlenses or concave mirrors and by which the losses of light which occuron entering and leaving the lens by absorption in the lens or by theimperfect reflection at the polished surfaces of the reflecting mira'orsare considerably lessened.

Another advantage lies in the simplicity of the apparatus in which theprism is used; the spectroscope consisting only of the prism, the slitand a magnifying glass for the examination of the image; and thespectrograph consists only of. the slit, the prism and a frame forholding the photographic plate. The cost of the apparatus iscorrespondingly lessened. Also the absence of the lenses of thecollimator and eye-piece get rid of the parasitical reflections whichfog the photographic plate in the ordinary spectrographs.

The invention will be described in connection with the accompanyingdrawings, wherein Figures 1 and 4 illustrate the prism and diagrams ofthe refractions and reflections therefrom; Fig. 2 illustratesdiagrammati- Hcally a spectrograph, with my prism ap plied, in plan;Fig. 3 is an end view of the apparatus illustrated in Fig. 2; Figs. 5and 6 illustrate diagrammatically in plan and longitudinal sectionrespectively a spectro- .scope in which my prism is applied; and

Fig. 7 illustrates a mounting for the spectrograph or the spectroscope.

The prism is formed (Fig. 1) of two spherical surfaces M N, P Q, ofsuitable.

- the refraction and dispersion of the rays land the other M N acts as areflecting mirror.

radii of curvature; the one P Q serves for For this purpose the surfaceM N is silvered or covered with a suitably chosen metallic film of highreflecting power. It is easily .shown that the conditions necessary forthe production of a pure spectrum can be realized by this system. Forlet A be the center of curvature of the front surface P Q and let theslit be placed at a point C such that 17. sin. '5'

sin. r

If the refracted rays P M and Q N are produced backward to meet at thepoint B and this point be taken as the center of.

curvature of the reflecting surface M N, then the rays falling normallyon this face will return on the same path, will undergo a refraction atI and Q of the same amount as on entering the prism and will meetwithout aberration at the point C.

If there is placed at C a source of monochromatic light, for which theindex of refraction 12' of the prism is greater than n, the angle ofrefraction 'r corresponding to the same constant angle 2' of incidencewill be smaller than 1" since The two reflected rays (which to avoidcomplicating the drawing are not shown) will not be exactly normal tothe surface M N, but will meet at a point near C but farther from A;this point also will be a true focus for all rays of this refrangibilitycoming from (7. In this way a spectrum will be formed it the slit isilluminated by a source which is not monochromatic. The spectrumproduced in this way will be accurately formed on a cylindrical surfaceof diameter R, equal to the radius of the front surface of the prism.

It can easily be shown that the angles A, B. and C are equal. From thetriangles P K A and Q K C,

i+A=i-|C; hence A=C. From the triangles A K P and hence A=B. The pointsA, B, C, Q, and I consequently lie on a circle, of which the point 0 isthe center; this will also be true for all points of the spectrum formedin the neighborhood of C. This shows the advan- Iatented Oct. 31, 1911.

tage of the arrangement which renders the spectral curve indepemlent ofthe law of dispersion of the material used, contrary to what takes placewith speclrographs with quartz lenses for instance, where the radius ofcurvature. of the focal surface. varies from one part of the. spectrluuto another. It is easy to calculate the. radius of curvature R of thereflecting surface. M N in terms of the radius 11 of the. retractingsurface 1 Q. If the distance 1 Q, is very small compared to It and R,

tzlt cos. A P li R cos. r.

If R is the radius of the focal curve R"=R cos. 2'.

The angles 2' and r (the latter is also equal to the an 'le of the )rismare connected b h I the expression sin. 5:11. sin. 1".

The diagrammatic Figs. 2 and 3 show a spectrograph constructed with thecurved prism. Fig. 2 is a plan and Fig. 3 a back view. The apparatusconsists simply of a rectangular box on which is placed at one end the.prism P and at the other end the slit (.3 and a curved frame X holding aplate or a photographic film stretched on a cylindrical frame. If theradius of curvature is small a cylindrical frame and a film should beused. With prisms of small angle and great focal length and focalsurfaces of large radius of curvature, the ordinary photographic platewould coincide practically with the focal curve. The two edges of theslit (l are adjusted by means of a screw r and the photographic frame Xon which the image V V of the spectrum is formed is placed below theslit C.

If it is desired to use the apparatus as a spectroscope, thephotographic frame is replaced by a metallic plate carrying an ordinarymagnifying glass to facilitate the examination of the rays of thespectrum.

The arrangement shown in Fig. 1 is known in optics as anauto-collimator, the rays coming from the slit C returning onthe samepath after undergoing a normal reflection at the points M and N. Thereare two drawbacks associated with this arrangement; in the first placethe spectrum (which lies on the circle passing through the points A, B,C, P, Q,) is inclined to the axis of the pencil coming from the prismand so necessitates placing the plate. in an oblique direction V V (Fig.2) with respect to the axis of the apparatus. In the second place, theslit must be placed at C above the frame V V in Fig. These drawbacks areobviated by the different path given to the rays as shown in Fig. 4. A Pis the radius of curvature of the first surface, B M that of the second.The slit is at the point C; as

shown above, all the points A, B, C, P, and Q lie on a circle ofdiameter A I. By suitably choosing the point C the ray which undergoesrefraction at Q may after reflection at M emerge normally to the firstsurface at P. Under these conditions the spectrum will be formed in theneighborhood of the point A and the photographic plate will be normal tothe pencil coming from the prism. Also the slit will be placed at theside of the apparatus at the point C, the spectrum being received at thepoint A upon an eye-piece or on a frame of large radius of curvature, oreven lane, since if the distance P V is sufficient y great, thecurvature may be negligible.

In Figs. 5 and 6 the invention is illustrated as embodied inaspectroscope, the spectroscope being constructed according to the planof Fig. t. The image can be examined by an eye-piece Y fixed to an arm 7pivoted below the prism P. An index 7 moving over a graduated scale ywill allow in this case wave lengths of the rays examined to be easilydetermined. Light. is admitted through a slit formed at C. lVhen longfocus prisms are used, the prism l and the plate or the eye-piece A maybe placed at the ends of a rod 3 of fixed length A P, these ends slidingin two guides or grooves 1 and 2 in a suitable base or platform at aright angle to each other. Under these circumstances the image willalways be in focus, the points P, C and A being the angular points of aright angled triangle inscribed in the circle of which A P is thediameter.

' Having now particularly described and ascertained the nature of mysaid invention and in what manner the same is to be performed, I declarethat what I claim is:

1. A prism for use in forming a spectrum, comprising a transparent bodyhaving two curved surfaces, one of which serves for refraction anddispersion of rays, and the other as a reflecting mirror for therefracted rays.

2. A prism for use in forming a spectrum, comprising a transparent bodyhaving two curved surfaces, one of which serves for refraction anddispersion of rays, and the other of which is provided With a reflectingmeans for reflecting the refracted rays.

3. A prism for use in forming a spectrum, comprising a transparent bodyhaving two curved surfaces, one of which serves for refraction anddispersion of rays, and the other as a reflecting mirror for therefracted rays, the said second surface being adapted to reflect saidrefracted rays Without refraction thereby.

4. A prism for use in forming a spectrum, comprising a transparent bodyhaving two curved surfaces, one of which serves for refraction anddispersion of rays, and the other as a reflecting mirror for therefracted rays, the said prism being adapted to reflect said refractedrays, and to refract saidreflected rays at the refracting surface.

5. A spectroscope or like instrument, comprising a part having a slittherein for passage of rays, a part on which the spectrum is projected,and a prism comprising a transparent body having two curved surfaces,one of which serves for refraction and dispersion of rays, and the otheras a reflecting mirror for the refracted rays, the centers of therefracting and reflecting surfaces of said prism, the slit for admissionof rays and the part on which the spectrum is formed all lying on thecircumference of one circle.

6. A spectroscope or like instrument, comprising a part having a slittherein for passage of rays, a sensitized plate upon which the spectrumis projected, and a prism comprising a transparent body having twocurved surfaces, one of which serves for refraction and dispersion ofrays, and the other as a reflecting mirror for the refracted rays, thecenters of the refracting and reflecting surfaces of said prism, theslit for admission of rays and the sensitized plate, all lying on thecircumference of one circle. i

7. A spectroscope'or like instrument, comprising a part having a slittherein for passage of rays, an eyepiece upon which the spectrum isprojected, and a prism comprising a transparent body having two curvedsurfaces, one of which serves for refraction and dispersion of rays, andthe other as a reflecting mirror for the refracted rays, the centers ofthe refracting and reflecting surfaces of said prism, the slit foradmission of rays and the eyepiece all lying on the circumference of onecircle.

8. A spectroscope or like instrument, comprising a part having a slittherein for passage of rays, a prism comprising a transparent bodyhaving two curved surfaces, one of which serves for refraction anddispersion of rays, and the other as a reflecting mirror for therefracted rays, the said refracted rays being reflected from said secondsurface without refraction, and a art on which the spectrum isprojected, sa1d part. being located adjacent the center of curvature ofthe refracting surface of the prism.

.9. A spectroscope or like instrument, comprising a part having a slittherein for passage of rays, a prism comprising a transparent bodyhaving two curved surfaces, one

of which serves for refraction and dispersion of rays, and the other asa reflecting mirror for the refracted rays, the said refracted raysbeing reflected from said second surface wlthout refraction, and asensitized plate upon which the spectrum is adapted to be projected,said plate being located adjacent the center of curvature of therefracting surface of the prism and at right angles to ghe raysreflected from said reflecting surace.

10. A spectroscope or like instrument, comprising a. part having a slittherein for passage of rays, a prism comprising a transparent bodyhaving two curved surfaces, one of whichserves for refraction anddispersion of rays, and the other as a reflecting mirror for therefracted rays, the said refracted rays being reflected from said secondsurface without refraction, an eyepiece pivoted to the instrument, uponwhich the spectrum is adapted to be projected, said eyepiece beinglocated adjacent the center of curvature of the refracting surface ofthe prism and a movable index with a fixed graduated are for determiningthe wave lengths of the rays.

11. A spectroscope or like instrument,

CHARLES FERYQ Witnesses H. o. Coxn,

RENi: BARDY.

